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The BBC News today reported that the “health hazards of obesity may have been grossly underestimated because we are not measuring the condition adequately”. Its website says that we should not focus on weight gain alone, but also look at how long it persists.

This news story was based on an analysis of data from the Framingham Heart Study, a long-running research project started in 1948 that went on to study participants for up to 48 years. As part of the study, researchers measured whether participants were obese every two years, as well as recording various aspects of their health. This new analysis found that the longer people stayed obese the greater their risk of dying from any cause (all-cause mortality), as well as cardiovascular diseases specifically.

This study further highlights the health risks of obesity. The researchers say that the duration of obesity is particularly important in today’s society where people are becoming obese at an earlier age. A healthy body mass index (BMI) is considered to be between 18.5 and 24.9, whereas obesity is classified as having a BMI above 30. People who are concerned about their weight can obtain help and advice from their GP.

Where did the story come from?

The study was carried out by researchers from Monash University, Australia. It was funded by an AusAID scholarship, a fellowship from VicHealth and the Australian National Health and Medical Research Council. The study was published in the peer-reviewed International Journal of Epidemiology.

The BBC News gave a top line review of this research and reported the research well.

What kind of research was this?

This was an analysis of a prospectivecohort study that had followed people for up to 48 years. The researchers were interested in seeing whether there was a specific association between mortality and the length of time a person was obese, rather than just the fact that they were obese.

It has been well established that being obese increases the risk of death and numerous health conditions, for example heart disease, diabetes and cancer. The researchers say that when quantifying the risks of numerous diseases the measures used have generally been body weight and BMI, which are related to the severity of obesity. However, the researchers wanted to know the role that duration of obesity plays, e.g. whether the risks would be the same for a person who had been obese for one year compared to a person who had been obese for 20 years. They refer to this factor as either one “obese year” or 20 “obese years”.

To understand the association the researchers assessed how the number of years lived with obesity related to the risk of all–cause mortality, death due to cardiovascular disease, cancer and other conditions.

What did the research involve?

The researchers used data from a long-running cohort study called the Framingham Heart Study. In 1948 this extensive cohort study enrolled 5,209 participants aged between 28 to 62 years, following them up for around 48 years. The participants had been examined at two-yearly intervals. The current study included those participants who were free from pre-existing diseases of diabetes, cardiovascular disease or cancer at the start of the study - 5,036 people in total.

The study recorded demographic and health behaviour variables such as age, educational level, country of birth, marital status, smoking status, number of cigarettes smoked per day, alcohol consumption and physical activity. A participant was considered obese if their BMI was more than 30 kg/m2. Among the chronic diseases that were regularly measured and included in the analysis were diabetes, cancer and cardiovascular disease (CVD) outcomes such as heart disease and stroke.

The researchers calculated the cumulative duration of obesity for each participant at each examination. As people who were borderline obese or overweight could have fluctuated over the course of the follow-up period, the researchers defined obese individuals as people who were obese at two consecutive examinations, i.e. continuously obese for at least two years. People could have multiple periods of obesity during follow-up (with weight loss in between). For these people, the researchers added all of their obese periods together to generate a cumulative score.

The researchers calculated a ‘time to event’ score for each individual, which represented either their survival time in days from study start to their death, their loss to follow-up or the end of the study (examination number 24, given in year 48 of the study).

For parts of the analysis the researchers grouped the duration of obesity into the following periods:

Short: 1 to 4.9 obese years

Medium: 5 to 14.9 obese years

*Long: *15 to 24.9 obese years

Over 25 obese years

What were the basic results?

The researchers found that 75% of the eligible study participants were not obese in any of the 24 examinations. Among participants who had two consecutive obese examinations, the average age of onset of obesity was around 50 years. The average number of years that this group lived with obesity was 13 years (time spent as obese ranged from 2 to 46 years).

The researchers then combined all the years of follow-up for the whole cohort. This resulted in 166,130 person years of follow-up. Over this time 3,397 (75%) of the participants died. Of the deaths, 39% were caused by CVD, 25% by cancer and 36% by other non-CVD and non-cancer causes.

The researchers adjusted their results within several models. The one used for the main results adjusted for the influence of sex, age at baseline, marital status, education level, country of birth, time-varying smoking, alcohol consumption and BMI.

Relative to people who had never been obese, the researchers calculated the increased risks of death due to any cause (all-cause mortality) during the study period:

Long duration of obesity increased the risk by 69% (95% CI 1.20 to 2.39).

Obesity for over 25 years increased the risk by 50% (95% CI 1.00 to 2.24).

They found that every two years living with obesity, relative to people who were never obese, resulted in a 6% increased risk of death due to any cause, a 7% increase in the risk of death following cardiovascular disease and a 3% increase in cancer-related mortality.

How did the researchers interpret the results?

The researchers said that “the number of years lived with obesity is directly associated with the risk of mortality; this needs to be taken into account when estimating its burden on mortality".

The researchers said that their study “confirmed that prior analyses examining the association between obesity and the risk of mortality” but “by only considering the severity of obesity and ignoring the duration of obesity may have underestimated the adverse effects of current obesity”.

Conclusion

This analysis of data from a prospective cohort study shows that duration of obesity is associated with mortality risk, particularly CVD-related mortality. The researchers said the key strength of this study was its long follow-up (up to 48 years), but they highlight that this is also a limitation due to the demographic and medical changes that have occurred since the study began. For example, they say that rates of obesity and type 2 diabetes were relatively low in 1948 when the study commenced, but that the contemporary obesity epidemic is characterised by a much earlier onset of obesity, which would mean that people today may have an even longer duration of obesity than the study population. Likewise, advances in medical treatments since 1996 (the last follow-up date in this study) may have affected the prevalence of CVD or cancer-related deaths.

The researchers also pointed out that for the people who were obese at baseline, there is no indication of when they became obese. Therefore the estimation of duration of obesity in these people may be imprecise.

Taking these limitations into consideration, the researchers said that in current and future studies the duration of subjects’ obesity needs to be taken into account in estimating the future life expectancy and burden of disease for the general population.

This research again highlights the health dangers of being obese. People who are obese and are looking for ways to lose weight can consult their GP for help and advice. Further research is needed to see whether weight loss following being obese lowers these risks over time.